1. Field of the Invention
The present invention is directed to particular tissue plasminogen activator (t-PA) variants having a modified structure that includes deleted amino acids within the protease domain of t-PA, which modification renders the variant more fibrin (or plasma clot) specific than wild-type (wt) t-PA.
2. Description of Background and Related Art
Plasminogen activators are enzymes that activate the zymogen plasminogen to generate the serine proteinase plasmin (by cleavage at Arg561-Val562) that degrades various proteins, including fibrin. Among the plasminogen activators studied are streptokinase, a bacterial protein, urokinase, an enzyme synthesized in the kidney and elsewhere and originally extracted from urine, and human tissue plasminogen activator (t-PA), an enzyme produced by the cells lining blood vessel walls.
The mechanism of action of each of these plasminogen activators differs: Streptokinase forms a complex with plasminogen or plasmin, generating plasminogen-activating activity, urokinase cleaves plasminogen directly, and t-PA, fibrin, and plasminogen all interact to yield maximum activity.
t-PA has been identified and described as a particularly important and potent new biological pharmaceutical agent that has shown extraordinary results in the treatment of vascular diseases, such as myocardial infarction, due in part to its high fibrin specificity and potent ability to dissolve blood clots in vivo.
Although the existence of t-PA prompted numerous investigations by several scientific groups, it was first identified as a substantially pure isolate from a natural source, and tested for requisite plasminogen activator activity in vivo, by Collen et al., U.S. Pat. No. 4,752,603 issued Jun. 21, 1988. See also Rijken et al., J. Biol. Chem., 256:7035 (1981).
Subsequently, t-PA was fully identified and characterized by underlying DNA sequence and deduced amino acid sequence based on successful work employing recombinant DNA technology resulting in large quantities of t-PA in a distinct milieu. This work was reported by Pennica et al., Nature, 301:214 (1983)) and in U.S. Pat. No. 4,766,075, issued 23 Aug. 1988.
Based on these disclosures, it seems now clear that the t-PA molecule contains five domains that have been defined with reference to homologous or otherwise similar structures identified in various other proteins such as trypsin, chymotrypsin, plasminogen, prothrombin, fibronectin, and epidermal growth factor (EGF). These domains have been designated, starting at the N-terminus of the amino acid sequence of t-PA, as 1) the finger region (F) that has variously been defined as including amino acids 1 to about 44, 2) the growth factor region (G) that has been variously defined as stretching from about amino acids 45 to 91 (based upon its homology with EGF), 3) kringle one (K1) that has been defined as stretching from about amino acid 92 to about amino acid 173, 4) kringle two (K2) that has been defined as stretching from about amino acid 180 to about amino acid 261, and 5) the so-called serine protease domain (P) that generally has been defined as stretching from about amino acid 264 to the C-terminal end of the molecule. These domains, which are situated generally adjacent to one another, or are separated by short "linker" regions, account for the entire amino acid sequence of from 1 to 527 amino acids of the putative mature form of t-PA.
Each domain has been described variously as contributing certain specific biologically significant properties. The finger domain has been characterized as containing a sequence of at least major importance for high binding affinity to fibrin. (This activity is thought important for the high specificity that t-PA displays with respect to clot lysis at the locus of a fibrin-rich thrombus.) The growth factor-like region likewise has been associated with cell surface binding activity. The kringle 2 region also has been strongly associated with fibrin binding and with the ability of fibrin to stimulate the activity of t-PA. The serine protease domain is responsible for the enzymatic cleavage of plasminogen to produce plasmin.
Despite the profound advantages associated with natural double-chain t-PA as a clot-dissolving agent and the advantage of natural single-chain pro-t-PA reported in EP Publ. No. 112,122, it is not believed that the natural protein necessarily represents the optimal t-PA agent under all circumstances. Therefore, several variants have been proposed or devised to enhance specific properties of t-PA. Certain of those variants address disadvantages associated with the use of natural t-PA in situations where an agent with a longer half-life or slower clearance rate would be preferred, e.g., in the treatment of deep-vein thrombosis and following reperfusion of an infarct victim, or where a single-chain agent is preferred.
For example, removal of a substantial portion or all of the finger domain results in a molecule with substantially diminished fibrin binding characteristics, albeit in return there is a decrease in the overall rate of clearance of the resultant entity. See WO 89/00197 published 12 Jan. 1989.
Variants are described in EPO Pat. Publ. No. 199,574 that have amino acid substitutions at the proteolytic cleavage sites at positions 275, 276, and 277. These variants, characterized preferentially as t-PA variants having an amino acid other than arginine at position 275, are referred to as protease-resistant one-chain t-PA variants in that, unlike natural t-PA, which can exist in either a one-chain or two-chain form, they are resistant to protease cleavage at position 275 and are therefore not converted metabolically in vivo into a two-chain form. This form is thought to have certain advantages biologically and commercially, in that it is more stable while the fibrin binding and fibrin stimulation are increased relative to two-chain t-PA. Furthermore, plasminogen activators are described that comprise one domain capable of interacting with fibrin and the protease domain of urokinase, with one embodiment being urokinase altered to make it less susceptible to forming two-chain urokinase. See WO 88/05081 published 14 Jul. 1988.
For further patent literature regarding modification of the protease cleavage site of t-PA, see, for example, EPO Pat. Nos. 241,209; EP 201,153 published Nov. 12, 1986; EP 233,013 published Aug. 19, 1987; EP 292,009 published Nov. 23, 1988; EP 293,936 published Dec. 7, 1988; and EP 293,934 published Dec. 7, 1988; and WO 88/10119.
Glycosylation mutants at 117-119, 184-186, and 448-450 exhibited higher specific activity as the mole percent carbohydrate was reduced. See EPO Pub. No. 227,462 published Jul. 1, 1987. This patent application additionally discloses using an assay of fibrin/fibrin degradation products and teaches that one may also modify the t-PA molecule at positions 272-280 or delete up to 25 amino acids from the C-terminus. Further, the t-PA mutants with Asn119, Ala186 and Asn450, which have the N-glycosylation sites selectively removed by DNA modification but contain residual O-linked carbohydrate, were found to be about two-fold as potent as melanoma t-PA in an in vitro lysis assay. See EPO Publ. No. 225,286 published Jun. 10, 1987.
Replacement of the amino acid at 449 of t-PA with any amino acid except arginine to modify the glycosylation site, as well as modification of Arg275 or deletion of the -3 to 91 region, is also taught. See WO 87/04722 published Aug. 13, 1987. An amino acid substitution at position 448 of t-PA is disclosed as desirable to remove glycosylation. See EPO Pub. No. 297,066 published Dec. 28, 1988. The combination of modifications at positions 448-450 and deletion of the N-terminal 1-82 amino acids is disclosed by WO 89/00191 published Jan. 12, 1989. Additionally, urokinase has been modified in the region of Asp302-Ser303-Thr304 to prevent glycosylation. See EPO Pub. No. 299,706 published 18 Jan. 1989.
However, alteration of the glycosylation sites, and in particular that at amino acid I-7, seems invariably to result in a molecule having affected solubility characteristics that may result additionally in an altered circulating half-life pattern and/or fibrin binding characteristics. See EPO Pat. Publ. No. 238,304, published 23 Sept. 1987.
When the growth factor domain of t-PA is deleted, the resultant variant is still active and binds to fibrin, as reported by A. J. van Zonneveld et al., Thrombos. Haemostas., 54 (1) 4 (1985). Various deletions in the growth factor domain have also been reported in the patent literature. See EPO Publ. No. 207,589 (substitutions and deletions between 51 and 87), EPO Publ. No. 241,209 (des-51-87), EPO Publ. No. 241,208 (des-51-87 and des-51-173), PCT 87/04722 (deletion of all or part o f the N-terminal 1-91), EPO Publ. No. 231,624 (all of growth factor domain deleted), and EPO Publ. No. 242,836 and Jap. Pat. Appl. Kokai No. 62-269688 (some or all of the growth factor domain deleted).
It has further been shown that t-PA can be modified both in the region of the first kringle domain and in the growth factor domain, resulting in increased circulatory half-life. See EPO Pat. Publ. No. 241,208 published Oct. 14, 1987. The region between amino acids 51 and 87, inclusive, can be deleted from t-PA to result in a variant having slower clearance from plasma. Browne et al., J. Biol. Chem., 263:1599-1602 (1988). Also, t-PA can be modified, without adverse biological effects, in the region of amino acids 67 to 69 of the mature, native t-PA, by deletion of certain amino acid residues or replacement of one or more amino acids with different amino acids. See EPO Pat. Publ. No. 240,334 published Oct. 7, 1987. See also WO 89/12681 published Dec. 28, 1989 on substitutions at residues between 63-72, especially 66 and 67, of wild-type t-PA.
A hybrid of t-PA/urokinase using the region of t-PA encompassing amino acids 273-527 is also disclosed. See EPO 290,118 published Nov. 9, 1988.
Serpin-resistant mutants of human t-PA with alterations in the protease domain, including d296-302 t-PA, R304S t-PA, and R304E t-PA, are disclosed in Madison et al., Nature, 339:721-724 (1989); see also the accompanying article by Dagmar Ringe in the same issue.
A general review of plasminogen activators and second-generation derivatives thereof can be found in Harris, Protein Engineering, 1:449-458 (1987). Other reviews of t-PA variants include Pannekoek et al., Fibrinolysis, 2:123-132 (1988) and Ross et al., in Annual Reports in Medicinal Chemistry. Vol. 23, Chapter 12 (1988).
It would be desirable to provide a t-PA molecule that, relative to wild-type t-PA, has a higher fibrin-stimulated (or a plasma clot-stimulated) activity than fibrinogen-stimulated (or plasma-stimulated) activity, i.e., is fibrin (or plasma clot) specific, so that it will act only at the site of the clot and not systemically.
Accordingly, it is an object of this invention to provide fibrin-specific t-PA molecules that exhibit improved therapeutic and pharmaceutical characteristics.
It is another object to provide for the treatment of conditions that admit the use of clot-dissolving agents that ar active only at the site of the clot and are useful at higher levels than other such agents.
These and other objects will be apparent to one of ordinary skill in the art.